Earthy bacteria faced with climate rap

Soil in Britain has lost an alarming amount of carbon over the past 25 years: more than enough to cancel out the country's reductions in carbon dioxide emissions.

The UK researchers who measured the loss claim its ultimate cause is climate change, which could be increasing the metabolism of soil bacteria so that they spit more carbon into the air. If true, this could feed more greenhouse gas into the atmosphere, causing more warming.

But others say the carbon change is due to changes in land use and precipitation patterns, which may not be linked to climate change.

Guy Kirk and colleagues at Cranfield University in Bedfordshire measured soil samples from almost 6,000 sites throughout England and Wales covering all types of land: grassland, peat land, uplands, woodlands, croplands and scrub. They measured the amount of organic carbon per gram of soil from each site twice between 1978 and 2003.

The net loss of carbon across the country was 13 million tonnes a year. That is roughly the amount by which Britain has reduced its carbon emissions from the base level that was set in 1990. It is only a small fraction of the 2,500 million tonnes of carbon thought to exist in the top 30 centimetres of British soil. But it is still a staggering amount.

"This is alarming," says earth-systems analyst Tim Lenton of the University of East Anglia in Norwich.

Ear to the ground

Losses occurred everywhere, irrespective of land use. This, say the authors, points to climate change as the likely culprit.

But the destination of the carbon is unclear. It may be leaching into water systems and deeper soils as bicarbonate and organic materials, or into the atmosphere as carbon dioxide, say the researchers. Their study did not use any tracers to determine where the carbon went.

It is crucial to understand the reason for the carbon loss, says Annette Freibauer at CarboEurope, an umbrella research group aiming to quantify carbon exchange between land and air. Until we know why soil is losing carbon it is difficult to know what conservation measures to implement.

Others argue that the study made certain assumptions that might not hold water. The team measured only the top 15 centimetres of the soil, where the majority of carbon changes occur, notes Lenton. But their extrapolation to the top 30 centimetres may make the results misleading, he says.

And the study did not keep a detailed history of land use in each site, with data on the amount of fertilization or whether animals grazed the land, for example.

Sexing up dirt

The team says its next step will be to look in detail at sites where land use has not changed, to pin down whether climate change is to blame for the carbon loss or not.

In the meantime, the researchers encourage policy-makers to think about conversion of some areas of farmland to forest as a means of stemming carbon loss from soils.

In the global warming debate, soil hasn't received enough attention, says Kirk. Most of the attention has focused on reducing fossil-fuel emissions, or studying whether the oceans and forests have the capacity to suck up extra carbon dioxide.

"People don't think soil is very sexy. They think it's boring old dirt," says Kirk. The researchers hope their study will change that focus.